Ceramics are widely used as structural members in industrial products as they are lightweight and hard and exhibit a high modulus of elasticity compared with metal materials. Boron carbide-containing ceramics are equipped with highest hardness and highest lightweight properties (bulk density: 2.5 g/cm3) among practical ceramics, and are expected to find utility, for example, as structural materials or the like of metal members which operate at a high speed. Recently, a process has been developed for obtaining a high-density sintered body as high as 95% or higher of the theoretical density by normal sintering (see Patent Document 1), thereby making it possible to stably provide high-purity dense boron carbide ceramics at low price. Wide-spread utilization of boron carbide ceramics is thus expected from now on.
On the other hand, the move toward larger operating machine members is remarkable in recent years. Concerning, for example, optical aligners for semiconductor fabrication systems, said optical aligners making use of a ceramic material, stages as operating machine members are increasing in size year after year to meet the ever-increasing size of silicon wafers. Ceramic materials for use in such operating machine members are, therefore, required to have a greater area. To meet such a requirement, there is a need for making greater the industrial facilities and processing equipment in ceramic fabrication processes. This need is, however, accompanied by an enormous investment in plant and equipment, thereby developing an extremely serious, practical problem that the economy of products will be impaired.
Under these circumstances, a technology is attracting interests. According to this technology, a large component of excellent characteristics is produced at low cost by forming a plurality of small ceramic members and bonding and integrating the resultant small ceramic members together into a larger one. On this technology, research and development work is under way at various research institutes and business enterprises as will be mentioned below. It is, however, difficult to strongly bond and integrate ceramic-made small members together with a high bond strength. Especially to use such a bonded ceramic body as a high-speed operating machine member for which the use of a boron carbide-containing ceramic is expected, a still higher bond strength is required, leading to an outstanding desire for the establishment of a still better bonding technology.
As a method for bonding ceramic members together into a ceramic structural body, it has heretofore been a common practice to bond them via one of various brazing materials or to bond them via a glass. In Patent Document 2, for example, with a view to obtaining an appropriate bond strength commensurate with the kind of a ceramic, a proposal is made to conduct the bonding of a metal and the ceramic with a silver-copper-indium based, active brazing filler metal. In Patent Document 3, a ceramic-bonding composition composed of an aluminum-silicon oxynitride glass is proposed for use upon bonding ceramics of the same kind or different kinds.
In Patent Document 4, it is proposed to heat faces of ceramic structural bodies, said faces being to be bonded together, to 660° C. or higher and to bond the ceramic structural bodies together via an aluminum material under heat or pressure. Proposed in Patent Document 5 is a bonding process that homogenizes a bonding layer between sintered ceramic bodies with the ceramic of the sintered ceramic bodies. Specifically, it is proposed to interpose aluminum metal between aluminum substrates, and subsequent to heating, to conduct oxidation treatment such that the aluminum metal becomes similar aluminum as that of the substrates. In Patent Document 6, a bonded body is proposed in which a member made of aluminum or an aluminum alloy and a ceramic are bonded together via a bonding layer. It is also disclosed that the strength of the bonding layer depends on the quantity of an intermetallic compound formed in the bonding layer and also that the quantity of the intermetallic compound can be controlled by specifying the content of copper in an aluminum matrix contained in the bonding layer. Ceramics to which the bonding can be applied by the above-described technologies include silicon nitride, silicon carbide, sialon, zirconia, and the like.
In Patent Document 7, there is proposed a process that, to strongly bond together silicon nitride ceramics exhibiting high characteristics as engineering ceramics, forms small members having shapes that their faces to be bonded together are fittable to each other, fills a silicon-containing paste between the faces to be fitted to each other, and converting the silicon to silicon nitride in a nitrogen atmosphere.